1. Field of the Invention
This invention relates to control devices for vehicular power transmitting apparatus. More particularly, it relates to a technology of preventing slippages in engaging devices due to locked one-way clutch incorporated in a vehicular power transmitting apparatus composed of a plurality of engaging devices including the one-way clutch.
2. Description of the Related Art
With a vehicular power transmitting apparatus operable to select a plurality of speed ratios, a control device has heretofore been employed in the power transmitting apparatus. The control device switches a method of selecting speed ratios depending on a running condition of a vehicle. Such a control device of an automatic transmission is taught in, for instance, Patent Publication 1 (Japanese Patent No. 2808923). Patent Publication 1 discloses the automatic transmission, adapted to perform a shifting based on a shifting map for selecting an optimum gear position i.e. speed position or shifting position depending on a vehicle speed representing a running direction of the vehicle and a depressing stroke of an accelerator pedal, with altering the shifting map based on an intake air quantity of an internal combustion engine representing an operating state of the vehicle.
Here, the vehicle speed, used as the running condition of the vehicle, is obtained upon detecting a rotation speed of an output shaft or a drive shaft with use of a rotation speed sensor. However, it becomes often difficult to obtain information related to a rotational direction, i.e. a running direction of the vehicle when acquiring the rotation speed, delivered from such a sensor, merely as a pulse signal. Therefore, the vehicle running direction has not been taken into consideration when executing the shifting. Under a circumstance where, for instance, an automatic gear position for a forward drive is selected with the shifting being executed using a shifting map for a forward drive, the shifting may be executed based on the shifting map for the forward drive even when the vehicle runs rearward.
In such an event, the shifting is executed based on an absolute value of the vehicle speed. Therefore, even with the vehicle running rearward at a high speed, the shifting is executed into a gear position for the forward drive at the high speed when the shifting map for the forward drive is used.
Meanwhile, one-way clutch has been widely used in part of the engaging devices of the power transmitting apparatus. The one-way clutch is operative to transfer a rotation in one direction while freewheeling i.e. idly rotating in the other direction. With such one-way clutches in use, the one-way clutches can be automatically disengaged or engaged, in contrast to a structure in which a so-called clutch-to-clutch shifting is performed with disengaging one one-way clutch and concurrently engaging the other one-way clutch. Therefore, it may suffice to allow only one one-way clutch to be engaged or disengaged, enabling the shifting to be executed in a simplified sequence.
With the power transmitting apparatus incorporating such one-way clutches, there exists a gear position that is established on the premise of freewheeling i.e. idle rotation of the one-way clutch. If a vehicle runs rearward when the gear position is selected with the one-way clutch placed under the freewheeling state, the one-way clutch is caused to rotate in a direction opposite to an original rotational direction of the one-way clutch, i.e. a direction in which the one-way clutch rotates when the vehicle runs forward. This causes the one-way clutch to be locked, with accompanying occurrence of confliction (a slippage caused in the engaging device engaging under a weakened engaging force as a result of a tie-up effect) between the one-way clutch and the other engaging element, causing a drop in durability of the engaging device.
Especially, as a result of the gear position for the forward drive being selected, if the shifting is performed based on the shifting map for the forward drive even when the vehicle runs rearward, slippages may occur in the engaging devices. This is because the shifting is executed based on an absolute value of the vehicle speed with a resultant unintentional shift from the gear position with the one-way clutch freewheeling to another gear position with the one-way clutch being locked.